Compositions for magnetic resonance imaging using a manganese oxalate

ABSTRACT

Methods and diagnostic compositions are disclosed for enhancing magnetic resonance imaging which utilize a paramagnetic metal cluster. Magnetic resonance contrast media containing Z +  [Mn 12  X 12  (OYR) 16l  (L) 4  ] - , where OYR is an oxyacid such as benzoic acid, acetic acid, methyl sulphonic acid, methyl phosphonic acid; L is a neutral donor such as water, alcohol, pyridine, or other amines; X is a chalcogen, such as O or S; and Z is a pharmaceutically acceptable counterion are disclosed. Z may also be a paramagnetic counter ion, such as a high spin metal cluster. The paramagnetic cluster may optionally be complexed or conjugated to a carrier compound capable of altering the paramagnetic metal cluster&#39;s biodistribution, increasing its selectivity of tumor localization, and amplifying its proton relaxivity.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. application Ser. No. 07/862,865 filed Apr.3, 1992, now U.S. Pat. No. 5,330,742 which is a CIP of U.S. applicationSer. No. 07/699,848, filed Aug. 5, 1991.

BACKGROUND OF THE INVENTION

This invention relates to compositions for improving magnetic resonanceimaging ("MRI"), magnetic resonance spectroscopy ("MRS"), and magneticresonance spectroscopy imaging ("MRSI"). More particularly, the presentinvention relates to multinuclear cluster compositions as magneticresonance contrast media ("MRCM").

The technique of MRI encompasses the detection of certain atomic nuclei(those possessing magnetic dipole moments) utilizing magnetic fields andradio-frequency radiation. It is similar in some respects to X-raycomputed tomography ("CT") in providing a cross-sectional display of thebody organ anatomy with excellent resolution of soft tissue detail. Thetechnique of MRI is advantageously non-invasive as it avoids the use ofionizing radiation.

The hydrogen atom, having a nucleus consisting of a single unpairedproton, has the strongest magnetic dipole moment of any nucleus. Sincehydrogen occurs in both water and lipids, it is abundant in the humanbody. Therefore, MRI is most commonly used to produce images based uponthe distribution density of protons and/or the relaxation times ofprotons in organs and tissues. Other nuclei having a net magnetic dipolemoment also exhibit a nuclear magnetic resonance phenomenon which may beused in MRI, MRS, and MRSI applications. Such nuclei include carbon-13(six protons and seven neutrons), fluorine-19 (9 protons and 10neutrons), sodium-23 (11 protons and 12 neutrons), and phosphorus-31 (15protons and 16 neutrons).

While the phenomenon of MRI was discovered in 1945, it is onlyrelatively recently that it has found application as a means of mappingthe internal structure of the body as a result of the originalsuggestion of Lauterbur (Nature, 242, 190-191 (1973)). The fundamentallack of any known hazard associated with the level of the magnetic andradio-frequency fields that are employed renders it possible to makerepeated scans on vulnerable individuals. Additionally, any scan planecan readily be selected, including transverse, coronal, and sagittalsections.

In an MRI experiment, the nuclei under study in a sample (e.g. protons,¹⁹ F, etc.) are irradiated with the appropriate radio-frequency (RF)energy in a controlled gradient magnetic field. These nuclei, as theyrelax, subsequently emit RF energy at a sharp resonance frequency. Theresonance frequency of the nuclei depends on the applied magnetic field.

According to known principles, nuclei with appropriate spin when placedin an applied magnetic field (B, expressed generally in units of gaussor Tesla (10⁴ gauss)) align in the direction of the field. In the caseof protons, these nuclei precess at a frequency, F, of 42.6 MHz at afield strength of 1 Tesla. At this frequency, an RF pulse of radiationwill excite the nuclei and can be considered to tip the netmagnetization out of the field direction, the extent of this rotationbeing determined by the pulse, duration and energy. After the RF pulse,the nuclei "relax" or return to equilibrium with the magnetic field,emitting radiation at the resonant frequency. The decay of the emittedradiation is characterized by two relaxation times, T₁ and T₂. T₁ is thespin-lattice relaxation time or longitudinal relaxation time, that is,the time taken by the nuclei to return to equilibrium along thedirection of the externally applied magnetic field. T₂ is the spin-spinrelaxation time associated with the dephasing of the initially coherentprecession of individual proton spins. These relaxation times have beenestablished for various fluids, organs, and tissues in different speciesof mammals.

For protons and other suitable nuclei, the relaxation times T₁ and T₂are influenced by the environment of the nuclei (e.g., viscosity,temperature, and the like). These two relaxation phenomena areessentially mechanisms whereby the initially imparted radio-frequencyenergy is dissipated to the surrounding environment. The rate of thisenergy loss or relaxation can be influenced by certain molecules orother nuclei which are paramagnetic. Chemical compounds incorporatingthese paramagnetic molecules or nuclei may substantially alter the T₁and T₂ values for nearby nuclei having a magnetic dipole moment. Theextent of the paramagnetic effect of the given chemical compound is afunction of the environment within which it finds itself.

In MRI, scanning planes and sliced thicknesses can be selected. Thisselection permits high quality transverse, coronal and sagittal imagesto be obtained directly. The absence of any moving parts in MRIequipment promotes a high reliability. It is believed that MRI has agreater potential than CT for the selective examination of tissuecharacteristics. The reason for this being that in CT, X-ray attenuationand coefficients alone determine image contrast, whereas at least fourseparate variables (T₁, T₂, proton density, and flow) may contribute tothe MRI signal. For example, it has been shown (Damadian, Science, 171,1151 (1971)) that the values of the T₁ and T₂ relaxation in tissues aregenerally longer by about a factor of two (2) in excised specimens ofneoplastic tissue compared with the host tissue.

By reason of its sensitivity to subtle physiochemical differencesbetween organs and/or tissues, it is believed that MRI may be capable ofdifferentiating different tissue types and in detecting diseases whichinduce physicochemical changes that may not be detected by X-ray or CTwhich are only sensitive to differences in the electron density oftissue.

From the foregoing, it would be a significant advancement in the art toprovide physiologically compatible MRCM for enhancing images of bodyorgans and tissues.

Such MRCM are disclosed and claimed herein.

SUMMARY OF THE INVENTION

The present invention provides methods and diagnostic compositions forimproved magnetic resonance imaging utilizing high spin paramagneticmultinuclear clusters. As used herein, the term multinuclear clustersinclude metal clusters having two or more paramagnetic metal atoms. Thehigh spin multinuclear clusters preferably have a spin valuesignificantly greater than that of a single metal atom. Typicalparamagnetic metal atoms which can be included in the clusters are Ta,Cr, W, Mn, Fe, Co, Ni, Cu, Pr, Nd, Sm, Y, Gd, Tb, Dy, Ho, and Er.

The metal clusters included in the diagnostic compositions may be ionic(anionic, cationic, or zwitterionic) or non-ionic. One currentlypreferred diagnostic composition within the scope of the presentinvention includes derivatives of the neutral metal cluster Mn₁₂ X₁₂(OYR)₁₆ (L)₄, where OYR is an oxyacid such as benzoic acid, acetic acid,methyl sulphonic acid, methyl phosphonic acid; L=a neutral donor such aswater, alcohol, pyridine, or other amines; and X is a chalcogen, such asO or S. A specific example of the forgoing neutral metal cluster is Mn₁₂O₁₂ (O₂ CPh)₁₆ (H₂ O)₄. Such metal clusters are formulated intophysiologically tolerable diagnostic compositions.

Also disclosed are methods of performing MR diagnostic procedures whichinvolve administering to a warm-blooded animal a diagnosticallyeffective amount of the above-described MRCM diagnostic compositionscontaining a suitable metal cluster and then exposing the warm-bloodedanimal to a MR procedure.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods and diagnostic compositions forimproved magnetic resonance imaging utilizing paramagnetic multinuclearclusters. Possible multinuclear clusters for use in the diagnosticcompositions of the present invention and methods of preparing theclusters are described in copending patent application Ser. No.07/699,848, titled "Heavy Metal Clusters For Use As Imaging Agents,"which is incorporated herein by reference. Such paramagnetic clustersrepresent the active ingredient of the disclosed diagnosticcompositions.

Metal clusters are known in the art (F. A. Cotton, G. Wilkinson,Advanced Inorganic Chemisty, 4th Edition, Wiley & Sons, 1980,1080-1112). Cotton and Wilkinson state that "A metal atom cluster may bedefined as a group of two or more metal atoms in which there issubstantial and direct bonding between the metal atoms." These clustershave found the greatest application either as catalysts or as models formetal surface-catalyzed reactions. Metal clusters of many paramagneticelements are known and, in many instances, cluster-like compounds inwhich metal-metal bonding is weak or non-existent (ordinary polynuclearcomplexes) are also known. See, for example, U.S. Pat. No. 4,832,877 toBino et al., WO 90/03190 to Ranney, and WO 91/14460 to Nycomed AS, whichare incorporated herein by reference.

One currently preferred diagnostic composition within the scope of thepresent invention includes derivatives of the neutral metal cluster Mn₁₂X₁₂ (OYR)₁₆ (L)₄, where OYR is an oxyacid such as benzoic acid, aceticacid, methyl sulphonic acid, methyl phosphonic acid; L is a neutraldonor such as water, alcohol, pyridine, or other amines; and X is achalcogen, such as O or S. A specific example of the forgoing neutralmetal cluster is Mn₁₂ O₁₂ (O₂ CPh)₁₆ (H₂ O)₄, (X). This cluster may beprepared according to the procedure reported by Peter D. W. Boyd, etal., "Potential Building Blocks for Molecular Ferromagnets: [Mn₁₂ O₁₂(O₂ CPh)₁₆ (H₂ O)₄ ] with a S=14 Ground State," Journal of the AmericanChemical Society, Vol. 110, pp. 8537-8539 (1988). Reduction of Mn₁₂ O₁₂(O₂ CPh)₁₆ (H₂ O)₄ with iodide yields a high spin anion. See Abstractsof 203rd Meeting of the American Chemical Society, San Francisco,Calif., Apr. 5-10, 1992. The anion is an example of a high spin metalcluster having a spin value of 19/2. By comparison, Gd³⁺ has a spinvalue of 7/2.

[Mn₁₂ O₁₂ (O₂ CPh)₁₆ (H₂ O)₄ ] may be prepared with several differentcounter ions. For example, reduction with tetrapropyl ammonium and tetraphenyl phosphonium iodide salts yields (n-Pr₄ N)[Mn₁₂ O₁₂ (O₂ CPh)₁₆ (H₂O)₄ ] and (Ph₄ P)[Mn₁₂ O₁₂ (O₂ CPh)₁₆ (H₂ O)₄ ], respectively. Reactionwith KI in the presence of a paramagnetic cationic metal cluster, suchas [Mn₃ O(O₂ CR)₆ L₃ ], where R is Me or Ph and L is a neutral ligandsuch as water or pyridine may yield a composition with two high-spinmetal clusters, one cationic and one anionic.

For example, reaction of (I) with KI in the presence of (ClO₄) [Mn₃ O(O₂CMe)₆ (H₂ O)₃ ] gives [Mn₃ O(O₂ CMe)₆ (H₂ O)₃ ]⁺ [Mn₁₂ O₁₂ (O₂ CPh)₁₆(H₂ O)₄ ]⁻ plus potassium perchlorate and iodine.

The diagnostic compositions within the scope of the present inventionmay include a paramagnetic metal cluster that has been complexed withsolubilizing ligands or functional groups. For ionic clusters in anaqueous formulation, pharmaceutically acceptable counter ions arerequired and these counter ions may be paramagnetic. In addition, theparamagnetic metal cluster may optionally be complexed or conjugated tocarrier compounds which may increase the cluster's stability, alter itsbiodistribution, increase its selectivity of tumor localization, andamplify its proton relaxivity by slowing its rotational correlationtime. Typical carrier compounds which may be used in the presentinvention include polymeric or microspheric carriers, liposome carriers,and hydroxyapatite carriers.

Possible polymeric or microspheric carriers are described in WO 90/03190(incorporated by reference). Such carriers may be negatively charged,such as heparin, DTPA-dextrans, DTPA-hydroxyethyl starch, mono- orpolyphosphonates and succinylated-dextrans; neutral, such as dextran andhydroxyethyl starch; or positively charged, e.g., polyhydroxylatedquaternary amines. Where liposome carriers are utilized, theparamagnetic metal cluster may be internally entrapped by the liposomeor externally bound to the liposome. For hydroxyapatite carriers, theparamagnetic cluster may be internally entrapped by the hydroxyapatiteor externally bound to the hydroxyapatite particles.

The diagnostic compositions of this invention are preferably formulatedin biocompatible solubilizing media for enteral or parenteraladministration. The MRCM formulations may contain conventionalpharmaceutical carriers and excipients appropriate for the type ofadministration contemplated.

For example, parenteral formulations imaging advantageously contain asterile aqueous solution or suspension of a paramagnetic metal clusterMRCM according to this invention. Various techniques for preparingsuitable pharmaceutical solutions and suspensions are known in the art.Such solutions also may contain pharmaceutically acceptable buffers,stabilizers, antioxidants, and electrolytes, such as sodium chloride.Parenteral compositions may be injected directly or mixed with a largevolume parenteral composition for systemic administration.

Formulations for enteral administration may vary widely, as iswell-known in the art. In general, such formulations include adiagnostically effective amount of a paramagnetic metal cluster MRCM inan aqueous solution or suspension. Such enteral compositions mayoptionally include buffers, surfactants, adjuvants, thixotropic agents,and the like. Compositions for oral administration may also containflavoring agents and other ingredients for enhancing their organolepticqualities.

The diagnostic compositions within the scope of the present inventionare administered in doses effective to achieve the desired enhancementof the magnetic resonance image. Such doses may vary widely, dependingupon the degree of fluorination, the organs or tissues which are thesubject of the imaging procedure, the magnetic resonance imagingequipment being used, etc. Typical doses of the diagnostic compositionsare in the range from about 0.005 to about 20 mmol/kg body weight, andpreferably in the range from about 0.05 to about 5 mmol/kg body weight.

The diagnostic compositions of this invention are used in a conventionalmanner in magnetic resonance procedures. Compositions may beadministered in a sufficient amount to provide adequate visualization,to a warm-blooded animal either systemically or locally to an organ ortissues to be imaged, and the animal then subjected to the MRIprocedure. The compositions enhance the magnetic resonance imagesobtained by these procedures.

The invention may be embodied in other specific forms without departingfrom its spirit or essential characteristics. The described embodimentsare to be considered in all respects only as illustrative and notrestrictive. The scope of the invention is, therefore, indicated by theappended claims rather than by the foregoing description. All changeswhich come within the meaning and range of equivalency of the claims areto be embraced within their scope.

What is claimed is:
 1. A sterile diagnostic composition suitable for enteral or parenteral administration to a warm-blooded animal comprising:a diagnostically effective amount of a paramagnetic metal cluster having a formula: Z⁺ [Mn₁₂ X₁₂ (OYR)₁₆ (L)₄ ]⁻, where OYR is an oxyacid such as benzoic acid, acetic acid, methyl sulphonic acid, methyl phosphonic acid; L is a neutral donor such as water, alcohol, pyridine, or other amines; X is a chalcogen, such as O or S; and Z is a pharmaceutically acceptable counterion; and a pharmaceutically acceptable carrier.
 2. A diagnostic composition as defined in claim 1, wherein Z is a paramagnetic counterion.
 3. A diagnostic composition as defined in claim 2, wherein Z is [Mn₃ O(O₂ CR)₆ L₃ ]⁺, where R is Me or Ph and L is a neutral ligand.
 4. A diagnostic composition as defined in claim 3, wherein Z is [Mn₃ O(O₂ CMe)₆ (H₂ O)₃ ]⁺.
 5. A diagnostic composition as defined in claim 1, wherein the paramagnetic metal cluster is complexed or conjugated to a carrier compound capable of altering the paramagnetic metal cluster's biodistribution, increasing its selectivity of tumor localization, and amplifying its proton relaxivity.
 6. A diagnostic composition as defined in claim 5, wherein the carrier compound comprises a polymeric carrier.
 7. A diagnostic composition as defined in claim 5, wherein the carrier compound comprises a microspheric carrier.
 8. A diagnostic composition as defined in claim 5, wherein the carrier compound comprises a liposome carrier.
 9. A diagnostic composition as defined in claim 5, wherein the carrier compound comprises a hydroxyapatite carrier. 